First of all it's very dangerous for her to wear a halter turned out. She could try to scratch her head and get it caught on something.

If she were mine I would:

Get her into the corral (round pen) Don't try to catch her. Just start exercising her in there. Keep her going, do not let her stop. Do this until she looks like she is getting tired and try to stop. Ask her to stop, hold your hand up like you are going to pet her forehead. Start walking up to her. When she takes off, go back to exercising her. Then ask again, keep doing this until she stands still. She will eventually (maybe a few days) come to you when you ask her to stop and hold your hand up. She'll get too tired to argue with you anymore. (the length of time , all depends on her) Only start this if you have the day to spend.

You might have to let her live in there, depends on how stubborn she wants to be. You cannot give up though, it will only make it worse. So be prepared to feed and water her in there. Take out all the food & water when you work with her.

If she is smart she just might come to you in the corral (cause she knows she is caught) and not out in the pasture.

When you do get her caught and she stands for you, just pet her and walk away. She will
have this look like "what the heck" and realize it wasn't all bad. She mat even follow you as you walk away. It will be easier the next time. If it were me I'd keep doing this until there is no question she will stand for you, let it be the new normal behavior before I would turn her back out. If this is not possible and you have to corral her every time she might just get smarter about corralling and you will be doing the same thing in a larger area.

There are loads of other methods , but they are too harsh for me. I want my horses to do what I ask cause they trust me, not because they fear me.

Just another note, if every time a horse is caught the person does something to them they don't like (trims, deworm, shots, ride etc...) The horse will not want to come to that person anymore. So you have to mix it up. Catch them just to turn'em loose or to grain or brush'em. 99 percent of the horses are not born ornery or bad, we make'em that way by poor communication skills. Just put yourself in their shoes (lol) and work with them like you would want to be worked with.

I have had to do this on a couple of horses that I bought, only one kept it up for more than a day. She was tough, it took about a week with her. They all came around and I never had another problem with any of them, using this method.

Some one else may have a better method out there, this is just what works for me.
Good luck, keep us posted.